1. Field of the Invention
The present invention relates to a radiation detection device and a radiation detection method in a nuclear medical diagnosis apparatus.
2. Description of the Related Art
Heretofore, there has been provided a nuclear medical diagnosis apparatus wherein a subject is dosed with a drug which is labeled with a radioisotope (hereinbelow, sometimes abbreviated to “RI”), and the state of an RI distribution in the body of the subject is imaged on the basis of a result which is obtained by sensing and measuring gamma rays emitted from the RI, by means of a radiation detection unit. In particular, an SPECT (Single Photon Emission Computed Tomography) apparatus has been extensively known as an apparatus or means for radiographing the above image as a three-dimensional distribution image (tomogram). Owing to such an image, the user of the apparatus or an operator can confirm the situation of the interior of the body of the subject for example, a morbid part, a blood stream or a fatty-acid metabolic rate) without resorting to surgical means.
Besides, there has been known a PET (Positron Emission Tomography) apparatus wherein a plurality of such radiation detection units are included, and a positron emitting nuclide is utilized as the radioisotope, so as to image a pair of gamma rays which are emitted in directions of 180 degrees at the annihilation of a positron by combining with an electron, and which are coincidentally detected by the plurality of radiation detection units (as a coincidental counting measurement, or the coincidence acquisition of the gamma rays). Incidentally, there has also been known a so-called “combined SPECT/PET apparatus” which is capable of implementing both PET and SPECT by an identical system.
By the way, in the above apparatus, a suitable energy range (or energy window) is usually set, whereby only gamma rays having energy levels within the range are acquired. Here, the suitable energy range is set as, for example, “energy of and above a Compton edge”, and all the gamma rays corresponding thereto are acquired to form the basis of the imaging. Thus, gamma rays having given rise to a photoelectric effect can be utilized as principal basic data on the occasion of the imaging.
Herein, the radiation detection unit fundamentally has the function of receiving the incidence of the gamma rays, and converting the gamma rays into electric signals (easy of handling) while mirroring the incident positions and energy levels thereof. The practicable aspects of the radiation detection unit are of two broad sorts called a “scintillation camera” and a “semiconductor detector”.
The scintillation camera is chiefly constituted by a scintillator (made of, for example, an NaI crystal, BGO, or LSO) and a photo multiplier tube (PMT). According to this, the gamma rays incident on the scintillator are converted into light signals, which are converted into the electric signals by the photo multiplier tube. On the other hand, the semiconductor detector is so constructed that a plurality of semiconductor detection elements (of, for example, CdTe or CdZnTe) in which the incidence of the gamma rays contributes to the creation of charges (that is, the generation of the electric signals) are arrayed, for example, planarly (in the shape of a matrix) and discretely.
Meanwhile, in the nuclear medical diagnosis apparatus as explained above, a theme to be stated below is generally existent. It is to enhance the efficiency of the gamma-ray acquisition in the radiation detection unit. The reason therefor is that, since the result of the gamma-ray acquisition forms the basis of the imaging as stated above, usually a higher acquisition efficiency is more favorable in order to guarantee the image quality and preciseness of the imaging.
In this regard, in a case where the scintillation camera is utilized as the radiation detection unit and where its scintillator is made of the BGO, LSO or the like, stopping power for the gamma rays is comparatively high. Therefore, most of the incident gamma rays give rise to the photoelectric effect within the scintillator, and the gamma rays can be acquired without being wasted (that is, all the energy can be imparted into the scintillator), so that the problem of the acquisition efficiency can be said less serious.
However, in a case where the semiconductor detector is used, the stopping power is comparatively lower than in the scintillator, and many of the incident gamma rays undergo transmission etc. without causing interactions with the detection elements. Besides, although it is unreasonable to go to the extent of saying that the gamma rays are transmitted through the detection elements without imparting any energy thereto, it can be said by way of example that the gamma rays are transmitted after causing Compton scattering, or that they give rise to the photoelectric effect after having caused the Compton scattering. That is, in the case of utilizing the semiconductor detector, it is usually difficult to impart the energy of all the gamma rays to the detection elements, and the acquisition efficiency for the gamma rays accordingly lowers. Therefore, the quality of the image of the gamma rays has also been affected. Incidentally, when the thickness of the semiconductor detection elements is enlarged in order to heighten the stopping power with the intention of eliminating such a drawback, the Compton scattering occurs a plurality of times, and it becomes impossible to specify the incident positions of the gamma rays.
Besides, even with the scintillation camera, in a case where the scintillator is made of NaI or the like, the stopping power is usually low, and hence, there has been the possibility that the same problem as stated above will be similarly posed.
By the way, such a fact becomes more problematic in a case where the gamma rays are high in energy. Besides, in such a case, an attempt to heighten an acquisition efficiency for the high-energy gamma rays is recognized as the general theme irrespective of the sorts of the scintillation camera and the semiconductor detector as stated at the beginning of this section.
The present invention has been made in view of the above circumstances, and has for its object to provide, in a radiation detection unit, a radiation detection device for a nuclear medical diagnosis apparatus as is capable of enhancing an acquisition efficiency for gamma rays of high energy.